Electric lamp failure indicator circuit

ABSTRACT

A core of magnetic material in the shape of a closed loop has three windings thereon. The first winding forms a circuit with a lamp to be monitored. As long as the lamp&#39;s filament is intact, whether or not the lamp is energized, this circuit can conduct current. The second winding is connected to a pulse generator to induce variations in the magnetic flux in the core. The third winding detects these variations in flux and a detection circuit connected to the third winding produces an indication if the variations are greater than a predetermined threshold. If the lamp is energized, the current in the first winding causes the core to be saturated with magnetic flux and thus the induced flux variations are below the detection threshold. If the lamp is unenergized, but intact, the current pulses in the second winding induce counter-pulses in the first winding which counteract the effect of the pulses in the second winding and thus the induced flux changes are below the detection threshold. If the lamp filament is broken, the pulses in the second winding will produce a sufficient change in flux in the core to be detected. Thus a defective lamp is detected whether or not it is energized. A second embodiment monitors two lamps with one core.

BACKGROUND

The present invention relates to electric lamp failure indicators andmore particularly to a warning device for providing an indication thatfailure has occurred, e.g. in a lamp which fulfils an importantfunction. The invention is especially, although not exclusively,applicable to front and rear lamps of motor vehicles to provide anindication to the driver, inside the vehicle, that such lamps arefunctioning correctly.

Although in the following description reference is made specifically tomotor vehicles, the invention is also applicable to apparatus andinstruments where one or more lamps are relied upon to provide importantinformation and where it is desired to avoid unrecognized failure ofsuch lamps.

Lamp failure indicators for use with front and rear lamps on motorvehicles have been proposed previously. In one system, a low valueelectrical resistor is connected in series with a lamp being monitored;the voltage drop across the resistor which results when the lamp isswitched on and working is combined with corresponding signals from theother lamp circuits to hold off a warning lamp. If any one lamp fails,the warning lamp lights. The disadvantage of this system is that thepresence of the resistor reduces the output available from each lamp andthe need for an expensive circuit arrangement to detect the smallsignals from said resistor.

In another known system, a current balancing relay is employed. Thecurrent taken by each lamp being monitored flows through a solenoid, thesolenoids being arranged so that the magnetic field produced by thecurrent of one lamp of a pair opposes the field produced by the otherlamp of the pair and thus a null field exists when all lamps areoperating. If a lamp fails, the magnetic field is no longer balanced anda magnetic reed switch is caused to be closed to switch on a warninglamp. For this approach to be successful, there must be a reasonableclose balance in the power taken by each lamp in a pair and this causesdifficulties due to the wide tolerance in lamp ratings.

It is a purpose of the present invention to overcome or minimize one ormore of the disadvantages of the above lamp failure indicators.

Both known systems described above can only indicate whether a lamp isoperating correctly or has failed when the lamp is switched on, but itis desirable to have an indication that the lamp has failed even when itis switched off, e.g. during the hours of daylight, in the case of amotor vehicle, when it is most convenient to replace the lamp.

SUMMARY

The present invention provides a device for indicating an electric lampfailure, comprising (i) a core of magnetic material in the form of aclosed loop, (ii) a first wire winding on the core, adapted to beconnected in circuit with the lamp in such a way that electric currentflowing through the lamp also flows through said winding, the number ofturns on the first wire winding being sufficient to result in magneticsaturation of the core, (iii) a second wire winding on the core, adaptedto be connected to an electrical circuit, which circuit is arranged togenerate electrical current pulses in the second winding, therebyinducing variations in magnetic flux density in the core, and (iv) athird wire winding on the core, adapted to be connected to an electricalcircuit capable of detecting the amplitude of a voltage or currentinduced in the third winding as a result of a change in flux density inthe core due to a current pulse in the second winding, thereby providingan indication of whether or not the lamp is operating. The three wirewindings may be composed of any suitable number of turns. Suitably themagnetic core is a ferrite and is of toroidal form. The magneticmaterial may exhibit linear characteristics or square loopcharacteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described hereinafter with reference to theaccompanying drawings in which:

FIG. 1 shows, in schematic form, one embodiment of the device of theinvention,

FIG. 2 shows a typical magnetic flux density -- magnetic field strengthloop for a core of linear magnetic material applied to the device of theinvention, and

FIG. 3 shows another embodiment of the device of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In one embodiment, the second winding is adapted to be connected to acircuit which produces current pulses through said winding in such adirection as to increase the field strength and hence the flux densityin the core, and the third winding is adapted to be connected to a pulseamplitude discriminator adapted to operate an indicating or alarm devicewhen the amplitude of the change in flux density in the core due to acurrent pulse in the second winding exceeds a certain preset level. Whenthe lamp current is flowing in the first winding and each of the secondand third windings is connected with its associated circuit, themagnetic core is biased to operate in the saturated region and underthese conditions, the electrical output from the third winding, due tothe pulse current in the second winding, is small. However, if the lamp,or associated circuit, should fail, so that there is no longer anycurrent flowing in the first winding, then the condition of magneticsaturation of the core, which was due to the lamp current flowing in thefirst winding, no longer holds. The core is then magneticallyunsaturated and the electrical output from the third winding becomes ofmuch greater amplitude. This increased amplitude is detected by thepulse amplitude discriminator which operates an indicator or alarm suchas a warning lamp, buzzer etc.

A second embodiment enables two lamps to be monitored using a pair offirst windings on a single magnetic core. Each of the windings isadapted to be connected in circuit with a lamp to be monitored andarranged so that, when the lamp is working, the current passing throughits associated winding is such as to result in magnetic saturation ofthe core, the pair of first windings being arranged so that, when bothlamps are operating, the magnetic field produced by the current in onewinding is opposed by that produced by the current in the other winding.The result is zero magnetic field due to the pair of first windings,when both lamps are operating. The current pulses passing through thesecond winding are arranged to be alternately negative pulses andpositive pulses. When both lamps are operating, the amplitude of theoutput from the third winding is large; if one lamp fails, then thecondition of opposing magnetic fields no longer exists and the corebecomes magnetically saturated. This saturation is in either a negativeor positive sense depending upon which lamp of the pair fails. When thisoccurs, the amplitude of the output from the third winding decreases.This change in amplitude is detected by the circuit associated with thethird winding which then operates an indicator or alarm.

The device of the invention is able to monitor the condition of a lampin its unenergized or energized state. In the case of the firstembodiment referred to above, the addition of a capacitor connected inparallel with the first winding and lamp is necessary to allow themonitoring of the lamp when this is unenergized; the second embodimentreferred to above enables monitoring of an energized or unenergized lampto be effected without any additional circuit components.

Referring to FIG. 1 of the drawings, a core comprising a toroid 1 of amagnetic material composed of a ferrite is provided with a first wirewinding 2, a second wire winding 3 and a third wire winding 4. The firstwinding 2 is connected in circuit with a lamp 5 which is to bemonitored, a battery 6 which powers the lamp and a switch 7 whichcontrols the lamp. The second winding 3 is connected to a current pulsegenerator 8 which provides current pulses in the second winding andthereby induces variations in magnetic flux density in the core 1. Thethird winding 4 is connected to a pulse amplitude discriminating circuit9, comprising a voltage comparator, which triggers an alarm or indicator10, such as a warning lamp or buzzer.

The operation of the device is best understood by reference to FIG. 2 ofthe drawings. In FIG. 2 there is shown a typical B-H loop for a linearmagnetic material composed of a ferrite from which the toroidal core 1of FIG. 1 is made, where B represents the magnetic flux density and Hrepresents the field strength. In the region 11, the flux density B isan approximately linear function of the field strength H and thematerial is said to be unsaturated. Outside this region, e.g. at regions12, the magnetic material of the core is said to be saturated andincreasing field strength results in only a very small increase in fluxdensity. The field strength H necessary to operate the material at anypoint on the B-H loop is provided by the current passing through thefirst winding 2 when the lamp 5 is operating. The field strengthproduced is proportional to the product of the number of turns of wirein the winding 2 and the current flowing through the winding. If thefield strength H is varied, the flux density B also varies and theamplitude of the variations of flux density depend on the variations infield strength and on the steady state operating point on the B-H loop.If the steady state operating point is in the linear region 11, then anyvariations in field strength will produce larger changes in flux densitythan would variations of the same amplitude if the operating point is inone of the regions 12 of magnetic saturation. In the embodiment shown inFIG. 1, variations in field strength of constant amplitude and rate ofchange are applied to the magnetic material by passing current pulsesfrom generator 8 through the second winding 3 on the core 1. When thelamp being monitored is operating, the current flowing through the firstwinding 2 of the core results in magnetic saturation of the core and thesteady state operating point on the B-H loop is at S. The amplitude ofthe corresponding change in flux density at this operating point issmall (reference numeral 13 in FIG. 2). If the lamp 5 fails, then nocurrent flows through the first winding 2 and the core 1 is no longermagnetically saturated and the steady state operating point changes to 0in FIG. 2. At this point, the amplitude of the change in flux densitydue to the current pulses passing through the second winding issignificantly greater than when the operating point was at S (seereference 14 of FIG. 2). The amplitude of the change in flux density issensed in the third winding 4 on the core voltage pulses appear in thewinding 4 and the amplitude of these pulses depends upon whether theoperating point is in the magnetically saturated or unsaturated regionof the material of core 1. A voltage comparator 9 is connected to thewinding 4 and when the lamp 5 fails, the increased amplitude of thevoltage pulses in winding 4, compared with the small amplitude of thepulses when the lamp is operating, is detected by the comparator andthis causes an alarm 10, e.g. a warning lamp, to be operated.

It is possible to monitor two lamps using a single core of magneticmaterial and such an embodiment is illustrated in FIG. 3 of thedrawings.

A core, comprising a toroid 15 of a magnetic material composed of aferrite, is provided with a pair of first wire windings 16 and 17connected together at one end, a second wire winding 18 and a third wirewinding 19. Winding 16 is connected via a lamp 20 which is to bemonitored to one terminal of battery 22 and winding 17 is also connectedto this terminal of the battery via another lamp 21 which is also to bemonitored. The junction of windings 16 and 17 is connected to the otherterminal of battery 22 through a switch 23. Switch 23 operates tocontrol the lamps 20 and 21. The second winding 18 is connected to acurrent pulse generator 24 which provides alternating negative pulsesand positive pulses in the second winding and thereby induces variationsin magnetic flux density in the core 15. The third winding 19 isconnected to a pulse amplitude discriminating circuit 25 which triggersan alarm 26. When switch 23 is closed, lamps 20 and 21 operate if theyare sound and current flows through the windings 16 and 17. Thesewindings are arranged so that the current in one is opposed by thecurrent through the other. The result is zero magnetic field due tothese windings in the core 15. This means that the operating point onthe B-H loop is at O in FIG. 2. In this condition the amplitude ofoutput voltage pulses from winding 19 is large. The number of turns oneach of the windings 16 and 17 is arranged so that in the absence of acurrent flowing through one of them, the current through the other wouldresult in magnetic saturation of the core, this saturation being in apositive sense in the case of one winding and in a negative sense in thecase of the other winding. If one or the other of lamps 20, 21 fails,current ceases to flow through the winding 16 or 17 associated with thefailed lamp; the core 15 then becomes magnetically saturated and theoperating point on the B-H loop for the material is at a point such as Sin FIG. 2. When this occurs, the amplitude of output voltage pulses fromwinding 19 decreases to a low level. This change in amplitude isdetected by the circuit 26 which operates alarm 26 to indicate that oneof lamps 20, 21 has failed.

Instead of using a single core 15 for the pair of lamps, two cores couldbe used with windings as in FIG. 1, but a single current pulse generatorand pulse discriminating circuit could be used for both wound cores.

The arrangements shown in FIGS. 1 and 3 can be used to provide anindication that a lamp has failed even when the lamp is not energized,i.e., when switches 7 and 23 are in the open position. To achieve thiswith the circuit of FIG. 1, a capacitor 27 must be connected in parallelwith winding 2 and lamp 5. When switch 7 is open, the resistance of theunenergized lamp 5 is low; the core 1 with its windings thereonfunctions as a transformer with the result that when the current pulsesfrom generator 8 pass through winding 3, they result in output voltagepulses from the winding 2 and these produce a current in the winding 2and through the lamp 5, the capacitor 27 allowing this current to pass.This current is of sufficient amplitude to reduce the flux density inthe core 1 (due to the field produced by the current in winding 3) to alow value and hence the output from the winding 4 is of low amplitude.If the lamp 5 fails, the current through the winding 2 ceases and sincethe field produced by the current pulses in winding 3 is now no longeropposed by the "transformer action" current in the lamp circuit, theflux change in the core 1 becomes large, resulting in an output ofgreater amplitude from the winding 4. This change in amplitude can bedetected by the circuit 9 and used to operate alarm 10.

In the case of the arrangement of FIG. 3, there is no need to employ acapacitor in the circuit when used to detect failure of an unenergizedlamp. In FIG. 3, the windings 16 and 17 and lamps 20 and 21 form aclosed circuit even when switch 23 is open. When current pulses fromgenerator 24 pass through winding 18, they result in output voltagepulses from the windings 16 and 17 which add and produce a current inthe windings 16 and 17 and through lamps 20 and 21. In the same way aswith the arrangement of FIG. 1, this current reduces the flux density ofthe core 15 to a low value and hence the output from the winding 19 isof low amplitude. If either of lamps 20, 21 fails, the current throughthe windings 16, 17 ceases and the flux change in the core 15 becomeslarge, resulting in an output of greater amplitude from the winding 19.This change in amplitude can be detected by the circuit 25 and used tooperate alarm 26.

Although the present invention is described herein with particularreference to specific details, it is not intended that such detailsshall be regarded as limitations upon the scope of the invention exceptinsofar as included in the accompanying claims.

We claim:
 1. A device for indicating an electric lamp failure,comprisingi. a core of magnetic material in the form of a closed loop,ii. a first winding on the core forming an electrical circuit with thelamp allowing electric current to flow through said lamp while said lampis operable, said current being either induced by said core when saidlamp is unenergized or supplied by a source of direct electric currentfor energizing said lamp when said lamp is energized, the number ofturns on the first winding being such as to result in magneticsaturation of the core when the lamp is energized, iii. a second windingon the core in circuit with an electrical circuit for generatingelectric current pulses in the second winding whereby variations inmagnetic flux density are induced in the core, and iv. a third windingon the core forming an electrical circuit capable of detecting theamplitude of a voltage or current induced in the third winding as aresult of a change in amplitude of the said variations in flux densityin the core occasioned by failure of the lamp.
 2. A device according toclaim 1 in which the second winding forms part of a circuit which isable to produce current pulses through said winding in a direction suchas to increase the field strength and hence the flux density in thecore, and the third winding forms part of a circuit including a pulseamplitude discriminator capable of operating an indicating or alarmdevice when the ampliutde of the change in flux density in the core dueto the current pulse in the second winding exceeds a certain presetlevel.
 3. A device according to claim 2 in which there is provided acapacitor connected in parallel with the first winding and lamp wherebythe electric circuit is formed when said sources does not energize thelamp.
 4. A device according to claim 2 in which there is provided a pairof first windings on a single magnetic core, each forming a circuit witha respective lamp to be monitored and arranged so that, when a lamp isoperable, the current passing through its associated winding is such asto result in magnetic saturation of the core, the pair of first windingsbeing arranged so that, when both said lamps are operable, the magneticfield produced by the current in one of said pair of first windings isopposed by that produced by the current in the other of said pair offirst windings, and the current pulses passing through said secondwinding being arranged to be alternatively negative pulses and positivepulses.
 5. A device according to claim 1 in which there is provided acapacitor connected in parallel with the first winding and lamp wherebythe electric circuit is formed when said source does not energize thelamp.
 6. A device according to claim 1 in which there is provided a pairof first windings on a single magnetic core, each forming a circuit witha respective lamp to be monitored and arranged so that, when a lamp isoperable, the current passing through its associated winding is such asto result in magnetic saturation of the core, the pair of first windingsbeing arranged so that, when both said lamps are operable, the magneticfield produced by the current in one of said pair of first windings isopposed by that produced by the current in the other of said pair offirst windings, and the current pulses passing through said secondwinding being arranged to be alternatively negative pulses and positivepulses.